In recent years with the increased attention focused on improvements in the atmospheric environment and global warming prevention, an evaporated fuel suppressing apparatus (canister) is used for the purpose of reducing the release of evaporated fuel gas into the atmosphere from a fuel tank of a gasoline internal combustion engine. In general, an adsorptive material such as activated carbon is filled into the apparatus, by which evaporated fuel is adsorbed and trapped. Further, during the running of an engine, combustion air is introduced into the canister to desorb the thus adsorbed evaporated fuel, thereby allowing the fuel to burn inside the engine.
However, it is known that, when activated carbon is used as it is to carry out the adsorption/desorption of evaporated fuel gas, an exothermic reaction takes place to rise in temperature on adsorption, whereas an endothermic reaction takes place to decrease in temperature on desorption, thus resulting in reduced performance of adsorption/desorption. It is, therefore, known that in order to solve the above problem, a heat storage material is used in combination or activated carbon is increased in specific heat. For example, it has been proposed that a heat storage material is used inside a porous material such as activated carbon (Patent Document 1) and that activated carbon is allowed to contain a liquid great in specific heat, thereby increasing the specific heat (Patent Document 2).    Patent Document 1: Japanese Published Unexamined Utility Model Application No. Sho-63-57351    Patent Document 2: Japanese Published Unexamined Patent Application No. Sho-64-36961
In recent years, in the United States, strict regulations have been imposed on evaporated fuel gas and the amount of fuel gas to be evaporated from a motor vehicle parking for 72 hours (DBL: diurnal breathing loss) has also been regulated. Thus, there is the necessity that not only fuel gas evaporated from a motor vehicle is simply subjected to adsorption/desorption but also that these regulations are complied with by suppressing fuel gas resulting from motor vehicles parking for a prolonged time which evaporates into the atmosphere.
The applicant of the present invention previously developed as a fuel-evaporation preventing apparatus capable of complying with the above regulations a fuel-evaporation preventing apparatus in which a second canister made up of honeycomb activated carbon is connected to a first canister and filed a patent application (Patent Document 3). The fuel-evaporation preventing apparatus is able to reduce the evaporation of fuel gas into the atmosphere even for a motor vehicle parking for a prolonged period of time only by connecting a small-sized second canister constituted with honeycomb activated carbon in series with a first canister, which is therefore useful.    Patent Document 3: Japanese Published Unexamined Patent Application No. Hei-10-37812
There has also been proposed a canister which combines an adsorbent having the adsorption capacity of greater than 35 g n-butane/L (liter) between vapor concentrations of 5 vol % and 50 vol % with an adsorbent having the adsorption capacity of less than 35 g n-butane/L (liter) between vapor concentrations of 5 vol % and 50 vol % (Patent Document 4).    Patent Document 4: U.S. Pat. No. 6,540,815
The canisters described in Patent Document 3 and Patent Document 4 are able to suppress a leakage amount of gasoline even for a motor vehicle parking for a prolonged period of time by connecting a second canister in series with a canister in which granular activated carbon is used. These Patent Documents have disclosed that honeycomb-shaped activated carbon is used in a second canister. However, the honeycomb-shaped body described in Patent Document 3 is produced by extrusion molding and easily broken. Further, Patent Document 4 has also described the use of a honeycomb structure, which is made of ceramic, and also easily broken although lower in air flow resistance.
In recent years, a request has been made for activated carbon which is greater in strength against vibration in view of being mounted on a motor vehicle, and a problem with strength has been particularly pointed out in a conventional activated carbon honeycomb structure. Further, where the activated carbon honeycomb structure is installed inside a canister, a sealing material such as an O-ring is needed, thus posing another problem that production costs are higher. On the other hand, there is known a canister into which plural types of activated carbon different in performance of adsorption/desorption are filled. This canister is constituted by filling activated carbon A great in adsorption amount of evaporated fuel and weak in retaining force into a first adsorbent layer (main chamber) and activated carbon moderate in adsorption amount of evaporated fuel and weak in retaining force into a second and subsequent adsorbent layers (auxiliary chambers) (Patent Document 5).    Patent Document 5: Japanese Published Unexamined Patent Application No. 2002-256989
The performance of a canister is indicated by a butane working capacity (BWC), which is the mean of an increase in a predetermined amount of n-butane adsorbed on a canister into which activated carbon is filled and a decrease of n-butane desorbed by air. However, in general, where a carbon material is granulated, carbonized and activated according to an ordinary method to produce activated carbon, the activated carbon is decreased in apparent density with an increase in performance of the BWC and decreased in specific heat accordingly.
In order to prevent the above problem, a method is available in which activated carbon powder is granulated with a binder and used as granular activated carbon. For example, there is known shaped activated carbon made up of wood-based granular activated carbon and bentonite clay and described that the activated carbon is used in a canister (Patent Document 6). However, the applicant of the present invention has examined in detail this shaped activated carbon, finding that the activated carbon is heavily affected for adsorptivity and adsorption speed by the binder and not necessarily satisfactory in performance.    Patent Document 6: Japanese Published Unexamined Patent Application No. Sho-63-242343
There is also disclosed a process for producing an adsorbent in which powdery activated carbon, bentonite, and an inorganic adhesive agent are mixed at a ratio of 40-70 to 10-30 to 10-40, then, water is added to a resultant mixture of 80-120% by weight with respect to the mixture and shaped into a filter form (Patent Document 7). However, as apparent from the examples, the adsorbent disclosed therein is a plate-like adsorbent shaped by using a mold, and the method disclosed in Patent Document 7 is unable to provide granular activated carbon usable in practice.    Patent Document 7: Japanese Published Unexamined Patent Application No. Sho-59-69146